1. Field of the Invention
The present invention relates to a cutting tip, and particularly to a plunge-cutting forming tip (for use with a turning tool; hereinafter the forming tip may be referred to merely as a tip) for plunge cutting while transferring onto a workpiece the contour (ridgeline) of a cutting edge.
2. Description of the Related Art
A circumferential surface of a bearing race (an outer circumferential surface of an inner race or an inner circumferential surface of an outer race) is subjected to plunge cutting in turning. A cutting tool is not fed along the axis of rotation of a workpiece (bearing race) during turning, but is fed radially. A plunge-cutting forming tip for use in such cutting differs in the shape of a cutting edge (the shape of a ridgeline of a cutting edge) according to individual type of workpiece, as is understood from the nature thereof.
For example, the plunge-cutting edge of a forming tip disclosed in Japanese Utility Model Application Laid-Open (kokai) No. 4-83503 has a profile to be transferred onto an outer circumferential surface of an inner race (workpiece). The forming tip is pressed against the circumferential surface of the workpiece to thereby plunge-cut, for example, a raceway groove for balls, a seal groove, or a chamfer of the circumferential surface. A chip breaker assuming a concave form is formed on a rake face through grinding so as to break chips produced during turning.
In plunge cutting of a raceway groove of a bearing and circumferential surfaces located at opposite sides of the raceway groove by means of such a forming tip, cutting of raceway groove begins first, followed by cutting of the circumferential surfaces together with the raceway groove. During cutting, the width and number of chips; i.e., the width of a cutting edge in contact with the workpiece and the number of points of contact with the workpiece, vary. Further, in general, such cutting produces relatively wide cutting chips and a large cutting force. Therefore, feed is set low, so that chips become relatively thin.
Since the rigidity of such a chip is poor, the direction of ejection of the chip becomes unstable, and thus the ejected chip tends to weave about. Further, the chip is less likely to break off. Accordingly, even when the ejected chip is curled, the chip does not break in an appropriate length. As a result, the chip tends to become excessively long while the direction of ejection of the chip is unstable. Such a chip may become entangled with a workpiece, potentially scratching a cut surface of the workpiece, or may become entangled with a tool or a chuck, potentially interrupting the continuous operation of a machine. Further, chip disposal has been difficult.
Since a chip breaker is formed on such a tip through grinding, the chip breaker fails to appropriately deform a chip. Even when a chip is coiled or coned, the coiled or coned chip hardly breaks and is wound coarsely, thus failing to reduce a volume thereof, with resultant poor chip disposal.
When, for example, a central raceway groove and seal grooves located at opposite sides of the central raceway groove are cut simultaneously by means of a forming tip, a chip is produced in association with cutting of the central raceway groove, and at the same time chips are produced in association with cutting of the seal grooves. Since chips are produced in a plurality of rows, they tend to become entangled with cut surfaces in a complicated manner or to become entangled with each other into the form of a bird's nest. Such entangled chips tend to become entangled with a chuck or to cause breakage of a cutting edge or stoppage of a machine. Thus, the conventional forming tip tends to involve entanglement of chips and chips in large lumps, causing poor chip disposal and impairing cutting efficiency.